1. Field of the Invention
The present invention relates to a radiation monitoring apparatus used for emission control or radiation control in a nuclear reactor facility, a spent nuclear fuel reprocessing facility, or the like.
2. Description of the Background Art
A radiation monitoring apparatus used in a nuclear reactor facility, a spent nuclear fuel reprocessing facility, or the like, is composed of: a plurality of radiation monitors for channels, each having a detector which detects radiation and a measurement section which measures a count rate from a detection signal pulse outputted from the radiation detector; and a testing apparatus having a test pulse generation section which inputs a test pulse to each radiation monitor, and a test pulse control section which controls the frequency of the test pulse and performs input switching between the detection signal pulse and the test pulse in the measurement section of the radiation monitor. The testing apparatus controls an input switch of the measurement section of the radiation monitor, to switch the input from the detection signal pulse to the test pulse, and conducts an input/output response test for measuring the linearity of output with respect to input (hereinafter, referred to as input output linearity), and an alarm test for confirming alarm operation accuracy from a count rate at an alarm operating point while changing the frequency of the test pulse in a ramp up linear, thereby confirming the soundness of each radiation monitor.
In confirmation of the accuracy of count rate indication response and the accuracy of alarm operation, the testing apparatus inputs a test pulse to the measurement section, and changes the oscillation frequency of the test pulse of the test pulse generation section in a step linear or a ramp up linear in accordance with a test item. It is noted that, during a test period, alarm is blocked as necessary before the test, so that alarm will not be outputted from the radiation monitoring apparatus to the outside, and the blocking is released after the test.
Each radiation monitor converts the measured count rate to an engineering value such as a dose rate as necessary, to obtain the radiation dose. High alarm is set for a radiation dose level higher than a normal background level, whereby high alarm is generated for abnormality in a dose rate in a management area of the above facility or radioactivity (proportional to a count rate) in a process system, to inform an operator, and system isolation can be automatically performed as necessary. In addition, low alarm is set for a radiation dose level lower than the normal background level, whereby low alarm is generated for loss of a detection signal or decrease in a count rate of a detection signal due to failure of the radiation monitor, to inform an operator.
On the other hand, since a count rate measured by the radiation monitor statistically varies, a time constant is automatically controlled in accordance with the count rate, whereby the count rate is measured so as to make a standard deviation constant, thus maintaining measurement accuracy. In addition, the measurement section is required to perform measurement so as to cover a wide range of count rates from about 10 cpm to about 107 cpm. In order to eliminate discontinuity due to range switching, a negative feedback circuit using an up-down counter is provided so as to automatically make the standard deviation constant over a wide range without range switching. Thus, a count rate measuring method that enables high-speed operation is employed.
The count rate measuring method using the up-down counter reads an accumulated value of differences between addition input and subtraction input in a constant cycle, to calculate a count rate. The addition input is a digital pulse generated by amplifying a detection signal pulse inputted to the measurement section from the radiation detector and performing pulse-height discrimination. The subtraction input is a digital pulse generated by performing frequency division and frequency synthesis for a clock pulse based on the accumulated value. Particularly, the count rate measuring method is characterized by capability to measure accurately over a range up to a high count rate.
Since a count rate responds by a time constant in the count rate measurement using the up-down counter, it takes a long time to conduct an input/output response test for measuring the input output linearity by inputting a test pulse for each decade, and to conduct an alarm test for confirming alarm operation by inputting a test pulse. Accordingly, the following invention is disclosed. That is, step input that changes a test pulse in a step linear and ramp input that changes a test pulse in a ramp up linear, are combined for each test item, the magnitude of the step linear change and the duration time of step are set in advance so as to be optimized, and the inclination and the duration time of the ramp signal are set in advance so as to be optimized, whereby an inputted test pulse is caused to quickly approach a target value by step input and caused to gradually approach a target value by ramp input. By thus controlling the test pulse, test time is reduced (for example, Patent Document 1).
In addition, the following invention is disclosed. That is, based on determination from the present value and a target value of a count rate of the radiation monitor, the test pulse control section controls switching of input of a test pulse so as to operate by either addition input or subtraction input (for example, Patent Document 2).
Patent Document 1: Japanese Laid-Open Patent Publication No. 10-260262 (Paragraph [0010] and FIGS. 2 and 3)
Patent Document 2: Japanese Laid-Open Patent Publication No. 2012-47559 (Paragraphs [0032] to [0038] and FIGS. 1 to 3)
In the invention disclosed in Patent Document 1, it is difficult to control a test pulse so as to reach a target value in a short time, and therefore, improvement is required to reduce the test time. Particularly, in a test item for a low count rate, since a time constant is inversely proportional to a count rate and it is necessary to wait for an accumulated value to change by the time constant, there is a problem that it takes a long time to conduct the test.
Also in the invention disclosed in Patent Document 2, particularly, in a test item for a low count rate, since a time constant is inversely proportional to a count rate and it is necessary to wait for an accumulated value to decrease by the time constant, there is a problem that it takes a long time to conduct the test.